Spark plug



D. W. GREGORY Jan. 22, 1946.

SPARK PLUG Filed March 6, 1943 .2 Sheets-Sheet 2 M m mw m. 0 4

DOUGLAS w. GREGORY EEEMFMHW Patented Jean, 1946 s PATENT OFFICE SPARK PLUG Douglas W. Gregory, Jackson Heights, N. in assignor to The Firestone Tire 8; Rubber Company, Akron, Ohio, a corporation of Ohio Application March 6, 1943, Serial No. 478,265

Claims. (01. 123-169) This invention relates to spark plugs such as commonly are used with internal combustion engines, and more especially it relates to improvements in the manner of securing the center electrode of the plug in place. 7

In spark plugs of the character mentioned, it is common practice to make the center electrode of two parts, which parts are welded end to end. One part, which may be called the tip, is composed of a suitable alloy, usually of high nickel content, so as to withstand the high temperatures incidental to use without becoming corroded or oxidized. When mounted in the usual porcelain holder, said tip portion of the electrode which is at the lower end thereof, preferably is received in a bore or axial recess of somewhat larger diameter to provide clearance around the electrode, whereby thermal expansion of the latter in use will not impose strains upon the porcelain. The upper end portion of the center electrode may be composed of any inexpensive metal such as steel, and this portion of the electrode is sealed to the porcelain, and usually has threaded engagement therewith by means of threads formed on the electrode and complemental threads molded into the central bore of the porcelain.

Heretofore the sealing of the center electrodes in conventional spark plugs has been effected in either of two ways, namely, (a) by means of a liquid sodium-silicate base cement, (b) by means of a powder packed around an upset flange formed integral with the electrode, said flange being seated upon a ledge disposed within the axial bore of the porcelain. In using the first (a) method, it was customary to fill the center electrode hole with cement under pressure after the tip portion of the electrode had been inserted in the bore of the porcelain up to the threaded end of the upper end portion of the electrode, after which the electrode was worked to its ultimate position by screwing said threaded end portion thereof into the threaded portion of the porcelain. This method was not entirely satisfactory for the reason that cement was forced down the bore of the porcelain and around the tip porion thereof, and required to be washed therefrom to prevent operational difliculties, such as cracking of the porcelain due to lessened room for expansion of the electrode tip, or accelerated corrosion due to said cement. In general, such washing was a poorly regulated means for removing excess cement from the electrode tip. In using the second (b) method the powder was packed about the electrode and tamped into place, and

This construction had the inherent disadvantage in that there was not positive contact beween the center electrode and the so-called terminal screw, and the ignition connection was required to be made on said screw.

The chief object of this invention is to providev an improved spark plug construction wherein the aforementioned disadvantages of prior constructions are overcome. More specifically the invention aims to provide means whereby cement may be forced into exact permanent position under pressure without requiring the subsequent washing of cement from around the tip portion of the electrode; to provide a construction that obviates the formation of air pockets in the cement; to lessen the strain on the internal threads of the porcelain insulator resulting from the tightening of the screw portion of the electrode struc ture; to provide a new and improved type of internal thread for porcelain insulators which is readil amenable to ceramic processing of the porcelain, and will result in an insulator of greater strength with a thread of increased holding and sealing possibilities; and to provide a porcelain insulator that is so constructed as to provide a natural path for the flow of excess cement to the exterior of the insulator, elsewhere than by way of the bore in which the electrode tip is.

received. Other objects will be manifest as the description proceeds.

Of the accompanying drawings:

Fig. 1 is a diametric section through a spark plug embodying the invention;

Fig. 2 is a fragmentary section of the spark plug shown in Fig. l, on a larger scale, showing the operation of mounting and cementing the center electrode in the porcelain insulator;

Fig. 3 is a fragmentary section of a modified form of the invention;

Figs. ito 8 inclusive are diametric sections through other embodiments of the invention; and

Figs. 9 to 11 inclusive are fragmentary sections of other embodiments of the invention showing modified constructions at the top thereof.

Referring first to Fig. 1 of the drawings, there is shown a spark plug assembly comprising the usual metal shell I0, and a porcelain insulator l I, the latter being mounted in said shell and held in spaced relation thereto by soft metal gaskets l2, I2 in the usual manner. Positioned axially within the porcelain ii is a center electrode that is designated as a whole by the numeral ll, said electrode being sealed in place by suitable or preferred cement H. The plug also includes the usual ground electrode 15 that is mounted on the shell It at the lower end thereof. The porcelain II is formed with an axial aperture or bore in which the electrode II is received, and the specific shape of said aperture and the shape of the electrode received therein constitute the novel features of the improved plug.

The axial bore of the insulator ii comprises a shallow, cylindrical recess or counterbore ll at the top thereof, an internally threaded portion is immediately below the counterbore, a relatively short, smooth cylindrical portion 2| below the threaded portion, a frusto-conical downwardly tapered or bevel portion 2! at the lower end of portion 20, and a smooth cyilndrical'portion 22 extending from the small end of tapered portion 2i to the lower end of the insulator. The cylindrical portion 22 is smallest in diameter of any part of the axial aperture, and its length, as shown, is at least half the length of the insulator. The internal threads of the portion is are multiple in character, are rounded at crown and root, and are of relatively steep pitch or lead. Furthermore the threads are rugged in character so as to withstand substantial pressure without fracture.

The electrode l3 comprises a tip portion 2| constituting somewhat more than the lower half of the electrode, said tip portion being composed of the usual or preferred electrode alloy. In the assembled spark plug the tip portion of the electrode is received within the cylindrical portion 22 of the insulator aperture, the relative diameters of electrode and aperture being such that there is a small but definite space between them, as shown in Fig. 2, to enable thermal expansion of the tip portion 24 without strain upon the porcelain. The upper end of the-said tip portion is welded to the lower end of a threaded structure 25, the weld being indicated at 26. Thestructure 25 may be formed of any inexpensive metal such as steel. The threaded portion 25 is formed exteriorly with threads that are shaped complemental to the internal threads in portion I! of the insulator aperture, that is, they have the same rounded profile in transverse section and are of the same steep pitch or lead. The threads on electrode portion 25 are multiple, but of smaller number than the threads in the insulator. As shown, the threads in the insulator aperture are eight in number whereas there are but four threads on the electrode portion 25. The electrode portion 25 is received in portion 19 of the insulator in the assembled condition of the plug, and the electrode threads at 25 are symmetrically arranged so that they are engaged by alternate threads in the insulator. Obviously the number of threads in the insulator and on the electrode may differ from those shown anddescribed, it being required only that the number of internal threads in the insulator be a multiple of the number of threads on the electrode. I

At the upper end of threaded portion 25 the electrode is formed with an integral flange 22, and therebeyond is formed with a threaded terminal portion 29. The diameter of the flange 28 is greater than the diameter of the recess II in the top of the insulator I i so that said flange will cover said recess in the flnished plug. The terminal portion 251s formed exteriorly with threads of standard gauge and is adapted to receive the usual nut (not shown) by means of which an ignition wire may be secured to the electrode by being clamped between said nut and the flange 20. A sweeper collar or washer 30 of metal is mountedupon the electrode tip 2|, said collar having a sliding fit thereon. While, as stated above, cylindrical portion 22 extends at least half the length of the insulator, it will be understood that this mode of cementing may be employed regardless of the length of the electrode used for feeding cement into the cement well. The extent of the proportions of relatively larger and smaller portions of the bore may therefore be varied and in some cases the reduced portion of the bore may be of only minor extent, as for example, the form of the invention illustrated in Fig. 6.

In the assembling of the spark plug from the several elements described, the sweeper collar II is positioned upon the electrode tip 24 at a distance from the weld 26 that is greater than the distance between weld and collar in the finished plug. Then cement is daubed on the electrode to cover all the surface thereof between said sweeper collar and the flange 28. The cement may be of any usual or preferred composition, and since it is not a part of the present invention, no specific formula for the cement need be set forth here. The cemented electrode is then inserted into the insulator aperture from the top thereof, the tip portion 24 of the electrode being received in portion 22 of said aperture. During this operation the sweeper collar 30 engages with and seats itself upon the tapered portion 2| of the insulator aperture, with the result that further movement of the electrode causes tip portion 24 thereof to move through said collar. This causes the cement that is immediately above said collar to be scraped from the electrode and to accumulate in the region 20 of the insulator aperture. By this time the threads on threaded portion 25 of the electrode engage with alternate internal threads of portion it of the insulator aperture, so that the electrode is worked into ultimate position by a twisting movement, any entrapped air escaping upwardly along the unoccupied screw threads of the insulator aperture. The latter screw threads ultimately become filled with the cement, and the cement also fills any interstices between the electrode threads and insulator aperture threads with which they engage. Surplus cement is forced to the top of the insulator whence it'readily may be removed. The cement fills the counterbore II at the top of the insulator, and this cement adheres to the flange 20 of the electrode when the latter is in its ultimate position, as shown in Fig. 1.

An alternative method of applying the cement to the plug is to insert the electrode therein so that the sweeper collar II is seated before the threaded portion 25 of the electrode enters the top of. the insulator aperture, and then to inject the proper amount of cement into the aperture, there being suiiicient space between the wall of the latter and the electrode to enable this to be done. The electrode is then worked into its ultimate position in the manner previously described.

From the foregoing it will be seen that the assembling of the improved plug readily is accomplished, and that cement is prevented from entering the lower aperture region 22 of the insulator. The feature of providing a relatively long pitch internal thread of coarse size in the insulator facilitates the manufacture thereof since it is practically impossible to manufacture porcelain insulators having exact leads. For this reason insulators having internal threads having fine stresses and strains are ikely to concentrate. The multiple character of t e threads shown utilizesthe feature that a multiple point bearing or engagement may be had thereby notwithstanding shrinkage variation characteristics of the porcelain material- The invention provides a superior spark plug,

and achieves the other objects set out in the foregoing statement of objects.

The embodiment of the invention shown in Fig. 3 is similar to that previously described, and diflers therefrom only in respect to the arrangement of the axial aperture in the porcelain, and the construction of the sweeper collar, the electrode otherwise being identical to the electrode shown in Figs. 1 and 2. Said sweeper collar, designated 33, is composed of relativelysoft, deformable metal, such as copper or brass or a suitable alloy of sufilciently high melting point, and is cast upon the tip portion 24 of the electrode immediately below the weld 26 that joinssaid tip portion to the threaded portion 25 of the electrode, the arrangement being such that the sweeper collar is fixedly secured against movement relatively of the electrode. Initially the collar 33 is molded substantially to spherical shape, as indicated by the dotted lines, Fig. 3.

The porcelain insulator, designated 35, has the same external configuration as the insulator Ii previously described, but differs therefrom in the shape of its axial aperture. Thus the upper threaded portion 36 o! the latter'has but half the number of threads of the porcelain ll previously described, that is, there are the same number of threads in the porcelain as there are on the threaded portion 25 or the electrode. Because of the steep pitch and multiple arrangement of the inter-engaged threads, there are relatively wide, continuous helical spaces between the same, which spaces are filled with cement in the finished plug. The axial aperture of the porcelainhas a tapered portion 31 at the juncture of its threaded portion 36 and a reduced lower portion 38 wherein the tip portion 24 of the electrode is received, said tapered portion 31 being steeper or more acute than tapered portion 2i of the embodiment previously described.

The porcelain 35 and electrode are assembled substantially in the same manner as in the embodiment previously described, with the cement ll binding the structures to each other. During the assembling of the plug, the sweeper collar 33 in spherical form seats in the tapered portion 31 of the porcelain aperture before the electrode is moved to its ultimate position, and provides a stop to prevent the flow of cement therebeyond and into portion 38 of the porcelain aperture. Continued axial movement of the electrode into the porcelain deforms the collar 33 to the shape shown in full lines in the drawings.

In the embodiment of the invention shown in pitch and relatively small threads are often found It is therefore Fig. 4, the electrode has a smooth tip portion 40 at its lower end, which tip portion is welded at 4| to a portion 42 of larger diameter than has its upper end portion provided with a flange 43, and a terminal threaded portion 44 for the reception of the usual terminal nut (not shown). Immediately below the flange IS the electrode portion 42 is formed with a single screw thread 45 of standard pitch, which thread extends at least a third of the length of the said portion 42. There is a sweeper collar slidably mounted upon the electrode tip 40. The porcelain, which is designated N, is formed with an axial recess within which said electrode is received, said recess including a lower portion I! that closely encircles but does not touch the electrode tip All; a cylindrical portion 48 thereabove that embraces the unthreaded portion 42 of the electrode but is substantially spaced thereabout, there being a tapered portion 49 at the juncture of portions 41 and I8, and an upper threaded portion to adapted'to receive the threaded portion 45 of the electrode. At the top of the porcelain the aperture therein includes a counterbore II. The threads of portion 50 difler from standard threads in that their crowns are cut of! or truncated, with the result that when electrode portion 45 is threaded into aperture portion 50 there is a recess between the roots of the threads of the electrode and the tops of the threads oi! the porcelain aperture. This recess is continuous and extends from the apertureregion 48 to the counterbore 5! whereby surplus cement I may escape upwardly to the top of the porcelain and be removed therefrom during the assembling of the plug. The cement ,may be applied in any suitable manner'as hereinbeiore explained. A metal washer 52 is mounted upon the electrode before assembling the plug, said washer being positioned between the electrode flange l3 and the top of the porcelain insulator in the finished plug. The electrode car. ries the sweeper collar 30 that seats on tapered portion I! of the porcelain aperture in the assembled condition of the plug.

In the embodiment of the invention shown in Fig. 5 a porcelain insulator I4 is formed with an axialibore comprising a counterbore I! at the top thereof, a smooth cylindrical portion 56 immediately therebelow, a threaded portion 51 immediately below portion 56, the bottom of said threaded portion merging with the large end of a-short tapered portion 58 that opens into a smooth cylindrical portion 59 constituting the lower end portion of the aperture. The threaded portion ll of said aperture is formed with relatively large dual threads of relatively steep pitch Received in the porcelain aperture is an electrode comprising a tip portion 8|! that is received in portion 58 of the aperture, and a generally smooth portion 6| thereabove that is formed at its lower end with a pair of outstanding lugs 82, 62, said lugs being disposed at diametrically opposite positions on the electrode and engaging respective threads of the threaded portion 51 of the porcelain. Above portion N the electrode includes a threaded portion 88 formed with a single thread of standard size, and above said portion 83 is a flange 64 and the usual threaded terminal portion SI for receiving a nut (not shown). The electrode carries a sweeper collar 86 that seats on tapered region 58 of the porcelain in the assembled. condition of the plug, and a washer 81 beneath the flange 84 that seats upon the'top of the porcelain. I

In the assembling of the plug, the lugs 0! enwiththetbreadedportimllotthepolcewherebytbeeleetrodekmo'ledasipositionasitismannailyrohted. Oeisappliedineithermannerhereinbetore servetoeiiectanintimatebondbetweenthecement and electrode, and the thread; in portion I! ottheporcelainapertureassureanintimate bond between cement and porcelain.

In the embodiment of the invention shown in Fig.6aporcelaininsulatorllisiormedwithan axial bore having acounterbore II at the top thereof, a relatively long, smooth, cylindrical portion 12 below said counterbore, a relatively short threaded portion 13 below portion 12, and a relatively short, smooth cylindrical portion ll at the lower end thereof. The thread of portion 13 is largeandroimdedandalsoisasingle thread. Receivable in the porcelain aperture is an electrode having a tip portion 15 receivable in portion 14 01 said aperture. Said tip portion is welded at Ii to an upper, and maior portion 11 that is receivable in portion 12 of the porcelain aperture, said electrode portion 11 being or substantially smaller diameter than said aperture to provide space therebetween for cement ll. Adjacent its lower end the portion ll 01' the electrode is formed with a short screw thread 1! that is complemental to screw thread I3 the porcelain aperture, said thread it extending once around electrode portion 'il. Near its upper end the electrode portion 11 is formed with screw threads 7! of standard pitch, this threaded portion of the electrode being'receivable in aperture portion 12 of the porcelain with ample clearance, and in the counterbore 1|, said threaded portion being surmounted by a flange II and the usual terminal thread I I. A metal washer 82 is disposed between the flange ll and the top of the porcelain. This plug is essentially similar to that shown in Fig. except that the meshing of threads 13, 1| serves to prevent the flow of cement I 4 into the portion H of the porcelain aperture, whereby the use of a sweeper collar is dispensed with.

In the embodiment of the invention shown in Fig. 7, a porcelain insulator N is formed with an axial aperture comprising a portion '5 of relativeLv short length and small diameter at the lower end thereof, a longer portion 88 0t cylindrical shape and larger diameter dismed thereabove and joining portion '5 by way of a short tapered portion 81, a threaded portion 88 above portion 86 and formed with dual threads of large size, rounded profile, and steep pitch, and a 001mterbore ll at the upper end of said aperture. Received in said aperture is an electrode comprising a tip portion 50 disposed in aperture portion Ii and having a sweeper collar Ii slidably mounted thereon and seatable upon tapered portion 01 of the aperture, said tip portion being welded at I! to a major portion 93 of larger diameter disposed within aperture portions It and I0. Hectrode portion 03 is formed with a pair of outstanding lugs 94, 94 at diametrically pposite points thereon, which lugs eng e with the respective threads of threaded portion '8 of the porcelain. At its upper end electrode portion ll isformedwithanintegralflange llandaterminal threaded portion 00, the bottom face of saidilangebeingformedwithaconcentriebeveled II bwflthatwfllmintothcm at the porcelaln in the finished p lmoumtedinlaidcomterboreliisa ot Humble metal. which washer hitothebolllloaato b mgedtherebyandtinntopmvidem withterminalthreadaiilatitstmpermd. The electrodeisiolmedwithanintearalflangellt onportlon lll,andhasametaicollar llillidablymountedontip portion Ill. Thediameter otflangelllandcollarlflisbut lighflysmaller threaded portion lllthere being a flange m' ontheelectrodebelowsaidthreaded portion-which flangebearsonthewasher Ill intheflniahediormoitheplug. Theunderside pm i iv therebetwem. Thearrangunmtissnehthatthewasherm bedetormedbytheflshtmingottheelectrode inflieporcelain'wha-ebyamperim-lealatthe uppermddflieporcdainapertnreiaeflectcd.

In the structure shown in Fig. 10, a porcelain insulator I24 is formed with an axial aperture I25 in which is received a cylindrical electrode member I26 of smaller diameter to provide space for a quantity of cement I4 therebetween. The upper end of the electrode is formed with the usual terminal threaded portion I21 and integral flange I28 below said threaded portion. Between the flange I28 and the top of the porcelain is a metal washer I29 having an axial aperture as invention clearly show various constructions by large in diameter as the porcelain aperture I25, and mounted upon the electrode I26 is a metal bushing I30 that extends through said washer and into the top of said aperture. The lower end of bushing I30 is tapered or beveled, the arrangement being such that the bushing will enter the which the cement employed about the electrode of a spark plug is readily positioned and retained in the desired local regions within the porcelain, whereby the objects of the invention are achieved.

Modification may be resorted to without departing from the spirit of the invention, or the porcelain aperture and compact the cement therein as the electrode is moved downwardly to position during the assembling of the plug.

In the structure shown in Fig. 11, a porcelain insulator I32 is formed with an axial aperture that is formed with standard screw threads I33 at its upper end, has an annular tapered region, I34, immediately therebeneath, which tapered region opens into a. smooth cylindrical region I35 at its lower and. smaller end. Received in the porcelain aperture is an electrode hereinshown as an assembled structure comprising an upper element consisting of a terminal threaded portion I36, an integral flange I31 therebeneath adapted to bear against the upper end of the porcelain, and a threaded portion I38 received in the threaded aperture portion I33 of the porcelain. The threaded portion I38 of the electrode is axially recessed from its lower end to receive a lower electrode element I39 therein with a force fit, v

the upper end of the element I39 being knurled as shown to assure a secure union with the superposed structure. Electrode portion I39 is disposed in aperture portion I of the porcelain, and is smaller in diameter than said aperture portion to provide a space in which cement I4 is placed. Mounted upon the electrode element I39 immediately below the threaded portion I38 of the upper electrode element I38 is a sweeper collar I40 that seats in the tapered portion I34 of the porcelain aperture to provide an effective seal against the cement I4 in the space therebelow. The collar is shown as being shaped complemental to its tapered seat I34, but originally the collar is cast upon the element I39 in spherical form, and is deformed to conical shape as the ,electrode is threaded axially into the porcelain. In this respect the collar is similar to collar 33, Fig. 3.

In the embodiments of the invention illustrated in Figs, 4, 5 and '7 the junction of the large and small portions of the center electrode form a shoulder confronting the sweeper collar. For example, in Fig. 4, a shoulder is formed at 4|, confronting sweeper collar 30. When the center electrode 42 expands due to motor heat, the shoulder at 4| is forced toward sweeper collar 30 compressing the cement against the sweeper collar and tightening the seal at this point. The degree of this compressive force can be controlled by varying the distance between the nearest point of positive engagement with the insulator and the shoulder at 4|, this variation being illustrated in Figs. 5 and '1. In Fig. 5 the point of positive engagement of the center electrode with the porcelain is at lugs 62 only a short distance above the shoulder formed by the junction of the smaller and larger electrodes, and expansion of the center electrode will result in only a scope thereof as defined by the appended claims.

I claim: l. A spark plug comprising a porcelain insulator having an axial aperture therethrough that is of smaller diameter at its lower end portion and larger diameter thereabove with a tapered region between the regions of different diameters, a one-piece metal electrode received in said aperture and spaced from the wall thereof to provide space for the reception of cementitious material in plastic form, the lower end portion of the electrode being of reduced diameter, a member mounted on the reduced end portion of the electrode and movable relatively thereof adapted to seat on the tapered region of. the porcelain aperture during the mounting of the electrode in said aperture, before said electrode moves to its ultimate position in the latter, thus preventing flow of cementitious material into the lower end portion of said aperture, and threaded means for effecting longitudinal movement of the electrode into the porcelain aperture during the assembling of the plug. 1

2. A spark plug comprising a porcelain insulator having an axial aperture therethrough that is of reduced diameter at its lower end portion and has a tapered region connecting said region of reduced diameter with the region of larger diameter thereabove, a one-piece metal electrode its ultimate position therein, thus obstructing flow of cementitious material into the lower end portion of said aperture as the electrode moves thereinto, and cooperating screw thread means on the electrode and in the porcelain aperture-to eifect longitudinal movement of the electrode in its entirety during assembling of the plug, said thread means being so constructed and arranged as to enable cementitious material to move therepast.

3. A spark plug as defined in claim 2 wherein the thread means are of steep pitch and rounded at crown and roof.

4. A spark plug as defined in claim 2 wherein the thread means consists of multiple threads of steep pitch.

5. A spark plug as defined in claim 2 wherein the thread means consists of a plurality of threads of steep pitch on the electrode, and the number of threads in the porcelain aperture i a multiple of the number of threads on the electrode.

6. A spark plug as defined in claim 2 wherein the thread means consists of multiple threads of steep pitch, the threads being relatively widely spaced apart axially of the electrode and insulator aperture to provide space therebetween for the flow of cementitious material.

7. A spark plug as defined in claim 2 wherein the thread means comprises a single thread of steep pitch formed in the porcelain, and a thread of complemental size and pitch extending once around the electrode.

8. A spark plug comprising a porcelain insulator having an axial aperture therethrough, a onepiece metal electrode received in said aperture, the latter being of smallest diameter at its lower end portion, and being larger thereabove to provide substantial space about the electrode, there being a tapered region in said aperture between the regions thereof of different diameter, a memher on the lower end portion of the electrode and movable relatively thereof adapted to seat on the tapered region of the porcelain aperture during mounting of the electrode, before the latter is moved to its ultimate position in the porcelain aperture, thus sealing of! the lower end poition of the porcelain aperture against the flow of cementitious material as the electrode moves thereinto, and a quantity of cement in said porcelain aperture above said member and about said electrode therein.

9. A combination as defined in claim 8 wherein the sealing member is a sweeper collar slidably mounted on the lower end portion of the electrode.

10. A combination as defined in claim 8 wherein the sealing member is a mass of soft metal fixed on the electrode and deformed into conformity with the tapered region of the porcelain aperture during the assembly of the plug.

ll. A spark plug comprising a porcelain insulator having an axial aperture therethrough, which aperture has regions of different diameters connected by an intermediate tapered region, an electrode received in said aperture, 2: charge of cement about said electrode in one region of said aperture, and a deformable metal collar secured on said electrode and shaped to conformity with said tapered region of the porcelain aperture to isolate the cement in one region of the latter.

12.1 3, spark plug comprising a porcelain insulator formed with an axial aperture therethrough, which aperture is of smaller diameter at its upper end portion to provide an internal shoulder, an electrode received in said aperture and formed with a flange abutting said shoulder, said electrode havin a reduced lower end portion, a quantity of cement around said electrode within said aperture, and a collar slidably mounted upon the reduced portion of the electrode and receivable within the porcelain aperture to confine the cement therein between said flange and said collar.

13. A spark plug comprising a porcelain insulator having an axial aperture therethrough, a

metal electrode received in said aperture, said aperture having a smaller diameter at its lower end adjacent the sparking end of the plug and a larger diameter thereabove to provide a substantial space about the electrode, there being a shouldered region in said porcelain insulator between said portions of diflerent diameter, said electrode having a lower portion of smaller diameter and an upper portion of larger diameter, the junction between said different diameters constituting a shoulder confronting the shoulder in said insulator, a quantity of cement in said aperture confined between said shoulders about said electrode for sealing the latter in said insulator, and means on said electrode positively engaging a portion of said insulator in a region thereof spaced above said shoulders, whereby axial elongation of said metal electrode resulting from heat of operation will cause the shoulder on said electrode to be forced toward the shoulder on said insulator, compressing said cement therebetween and tightening the seal between said electrode and said porcelain.

14. A spark plug comprising a porcelain insulator having an axial aperture therethrough, a metal electrode received in said aperture, portions of the latter being substantially larger in cliameter than the electrode for the reception of plustic ceinentitious material, a member mounted on the electrode and movable relatively thereof for preventing flow of cementitious material to the lower end of the porcelain aperture, and coopcrating screw thread means on the electrode and in the porcelain aperture to effect longitudinal movement of the electrode relatively of the porcelain during assembly of the plug, the electrode threads being formed adjacent the upper end thereof of conventional size, shape, and pitch, the porcelain aperture being formed with threads of complementary size and pitch, the last mentioned threads having their crowns truncated to provide a continuous passage for cementitious material at the roots of the electrode threads in the assembled plug.

15. A spark plug comprising a porcelain insulator having an axial aperture thcrethrough, which aperture is formed with a connterbore at the top thereof, a metal electrode received in said pertur a charge of cementitious material within the porcelain aperture about the electrode therein, means within said aperture preventing flow of the cementitious material to the lower end thereof, and means on electrode effecting sealing of the upper end of the porcelain aperture at said counterbore, said means comprising a metal washer on the electrode received in the counterbore in the porcelain, and a flange on the electrode formed on its under side with a concentric beveled boss engageable with the inner circumference of said washer.

DOUGLAS W. GREGORY. 

